Membrane switches, sometimes referred to as "touch contact switches" have achieved phenomenal success over the last decade due to their simplicity, reliability and very low cost. Generally, these switches include upper and lower flexible plastic films separated by an intermediate spacer film. The outwardly facing surface of the upper film, or a separate graphics film, bears a matrix of alpha-numeric or functional symbols representing the underlying switch matrix. The lower side of one film has a plurality of partly opaque complex conductor areas, one for each switch aligned with one of the indicia on the symbol matrix and also aligned with cooperating conductive areas on the other film. There have in the past been attempts to back light (light from an internal source positioned behind the switch) membrane switches but they have required specially designed conductive areas on the film to match the shape of each indicium on the individual switches to avoid shadowing or have required the use of transparent conductors. The specially designed opaque conductive areas have not performed satisfactorily, nor have switches with transparent conductors.
Moreover, since in these specially designed switches the conductive areas (which are opaque) are configured to have the shape of the transparent portion of the indicia or graphics displayed to the operator each conductive area is different, making the switch assembly extremely costly and even with such a costly switch assembly, shadow lines produced by the conductive areas still appear.
The membrane switches that have found commercial acceptance usually include a plurality of silk screened, printed or electro-deposited parallel conductive bars on the facing surfaces of the membrane films. The parallel bars on one film are perpendicularly related to the parallel bars on the other film and the films are spaced by a third spacer film having an aperture surrounding the aligned conductive areas on the films. One of the conductive areas has spaced input and output conductors and the other film has "short bars" so that upon touch depression of the short bar film switch the conductive bars on that film engage, and connect the input and output conductive bars on the other film, causing switch actuation. In one specific membrane switch that has been found reliable, the input and output conductor bars are arranged in circular configuration and several manufacturers presently make this specific type of membrane switch sub-assembly, including Sierra Corporation of Sylmar, Calif., Transparent Devices, Inc. of Westlake, Calif., and W. H. Brady Company of Milwaukee, Wis.
Membrane switches of this general type have achieved a considerable degree of success in office and business equipment such as calculators, copying machines and cash registers, and in a variety of other applications in which there is a readily available source of artificial ambient light. However, the membrane switch technology has not as yet achieved any significant degree of success in applications where a constant source of ambient light is not available, such as in outdoor and vehicular applications because it has not been possible thus far to adequately internally illuminate the switch indicia due to the shodow lines caused by the opaque conductors on the membrane films that block light.
It is the primary object of the present invention to provide an improved illuminated membrane switch.